Traditionally, the predominately used analog signal amplifiers have been the Class A, Class B and Class AB amplifiers, with class A amplifiers operating in the linear amplification region of a single transistor, class B amplifiers using two transistors, one for amplifying positive-going signals and the other for amplifying negative-going signals, and class AB amplifiers being a hybrid. A few of the desirable characteristics of an amplifier are high gain factor, low power consumption, low or no output signal distortion, and a large bandwidth.
The active components of a typical Class A amplifier include a single transistor which amplifies the input signal, and a resistor which provides for proper biasing of the circuit and shunts the emitter current to ground. The major drawback of the Class A amplifier is the excessive power consumption in the amplifier circuitry, required to bias the amplifying transistor to its linear operating point.
In a typical Class A amplifier, the bias current is always on, and is larger than the load current. The excessive current is dissipated within the amplifier circuitry, causing high power consumption. The high power consumption of a Class A amplifier becomes extremely undesirable when the amplifier is included within an integrated circuit chip.
The active components of a typical Class B amplifier include an NPN and a PNP transistor, wherein the emitters of the two transistors are connected. The NPN transistor amplifies the input signal during the positive swing of the signal, and the PNP transistor amplifies the input signal in its negative swing. Two major drawbacks of the Class B amplifier are high output distortion at the crossover point, and slower circuit operation. The slower circuit operation is due to the use of a PNP transistor which is normally slower in operation than an NPN transistor in the normal integrated circuit configuration.
The crossover distortion results from a lack of coincidence of the transistor characteristics of the NPN and the PNP transistors at the crossover point. In a typical operation, a change in voltage of 1.4 volts is required from the point that one transistor has been switched off to the point when the other transistor is switched on. Meanwhile, during this period, neither the NPN or the PNP transistor is active, resulting in distortion of the output signal.
The normally required 1.4 volts change in voltage Q may be reduced by using other devices, such as operational amplifiers in connection with the classic Class B circuitry, or by providing bias current, thereby shifting to class AB operation. Although such attempts have resulted in a reduction in crossover distortion, the resulting circuit has the major drawbacks of higher power consumption and more complex circuitry.
The active components of a typical Class AB amplifier may include the same active components as in a Class B amplifier, as well as other components such as diodes, resistors and transistors, with increased bias current for the two transistors to reduce crossover distortion. Similar to a Class B amplifier, a Class AB amplifier provides bidirectional output swings. Functionally, the Class AB amplifier may be viewed as a hybrid between the Class A and the Class B amplifiers. With an increase in power consumption, the Class AB amplifier may achieve a lower crossover distortion and higher speed than that of a Class B amplifier. The obvious disadvantage, however, would be an undesirably high power consumption. Further, the crossover distortion is increased and the speed is decreased as the bias current into the circuit is decreased.
Therefore, the Class AB amplifier provides the user with a compromise between low power consumption on one hand and low crossover distortion and high speed on the lQ other hand. Further, the crossover distortion, in a Class B or a Class AB amplifier, is accentuated as the circuit operates at higher frequencies.
Accordingly, the principal object of the present invention is to provide a high speed, high gain amplifier circuit to accomplish current amplification at a wide range of input signal frequencies, while keeping the power consumption at a minimal level and achieving minimal or no crossover distortion.